where优先级

select name from emply where id >5;

先找表from emply

再找条件 where id >5

最后打印 你想打印的字段

可以把select看成打印

优先级为from->where-->select

先找到表，然后where一条条数据过滤，发现符合的就扔给select，select再梳理出自己需要的数据进行打印。

#where
select id,name,age from employee where id > 7;

select name,post,salary from employee where post='teacher' and salary > 8000;

select name,salary from employee where salary >= 20000 and salary <= 30000;
select name,salary from employee where salary between 20000 and 30000;

select name,salary from employee where salary < 20000 or salary > 30000;
select name,salary from employee where salary not between 20000 and 30000;

select * from employee where age = 73 or age = 81 or age = 28;
select * from employee where age in (73,81,28); 选一个

select * from employee where post_comment is Null;  不可以=Null 要用is去判断
select * from employee where post_comment is not Null;

select * from employee where name like "jin%";  %等于后面任意匹配(即任意长度任意字符)
select * from employee where name like "jin___";(_只匹配一个任意字符)